``The flurry of international activity is particularly striking because regulators have been slow to act in the United States, where trading firms and investors have been hardest hit by a series of market disruptions, including the flash crash of 2010 and the runaway trading in August by Knight Capital that cost it $440 million in just hours. While the Securities and Exchange Commission is hosting a round table on the topic on Tuesday, the agency has not proposed any major new rules this year.''